Recent warming seen as unprecedented spike
By Summit Voice
SUMMIT COUNTY — Paleoclimatologists say they’ve managed to establish an accurate global temperature record going back about 11,000 years to the end of the last ice age, showing that the pace of warming during the past century is unprecedented.
There have been times during the Holocene when the Earth was warmer than it is now, but never before have temperatures spiked as dramatically, said Candace Major, program director in the National Science Foundation’s Division of Ocean Sciences.
“The last century stands out as the anomaly in this record of global temperature since the end of the last ice age,” Major said. “This research shows that we’ve experienced almost the same range of temperature change since the beginning of the industrial revolution as over the previous 11,000 years of Earth history — but this change happened a lot more quickly,” she added.
Analyzing data from 73 ice and sediment core monitoring sites around the world showed that the planet today is warmer than it’s been during 70 to 80 percent of the last 11,300 years.
“Extending the reconstruction of global temperatures back to the end of the last Ice Age puts today’s climate into a larger context,” said lead author Shaun Marcott, of Oregon State University.
“We already knew that on a global scale, Earth is warmer today than it was over much of the past 2,000 years,” Marcott says. “Now we know that it is warmer than most of the past 11,300 years.”
Of concern are projections of global temperature for the year 2100, when climate models evaluated by the Intergovernmental Panel on Climate Change show that temperatures will exceed the warmest temperatures during the 11,300-year period known as the Holocene under all plausible greenhouse gas emission scenarios.
Peter Clark, an OSU paleoclimatologist and co-author of the Science paper, said many previous temperature reconstructions were regional and not placed in a global context.
“When you just look at one part of the world, temperature history can be affected by regional climate processes like El Niño or monsoon variations,” Clark said. “But when you combine data from sites around the world, you can average out those regional anomalies and get a clear sense of the Earth’s global temperature history.”
What that history show is that, during the last 5,000 years, the Earth on average cooled about 1.3 degrees Fahrenheit — until the last 100 years, when it warmed about 1.3 degrees.
The largest changes were in the Northern Hemisphere, where there are more land masses and larger human populations than in the Southern Hemisphere.
Climate models project that global temperature will rise another 2.0 to 11.5 degrees by the end of this century, largely dependent on the magnitude of carbon emissions.
“What is most troubling is that this warming will be significantly greater than at any time during the past 11,300 years,” Clark said.
Marcott said that one of the natural factors affecting global temperatures during the last 11,300 years is a gradual change in the distribution of solar insolation linked with Earth’s position relative to the sun.
“During the warmest period of the Holocene, the Earth was positioned such that Northern Hemisphere summers warmed more,” he said. “As the Earth’s orientation changed, Northern Hemisphere summers became cooler, and we should now be near the bottom of this long-term cooling trend–but obviously, we’re not.”
The research team, which included Jeremy Shakun of Harvard and Alan Mix of OSU, primarily used fossils from ocean sediment cores and terrestrial archives to reconstruct the temperature history.
The chemical and physical characteristics of the fossils–including the species as well as their chemical composition and isotopic ratios–provide reliable proxy records for past temperatures by calibrating them to modern temperature records.
“The Earth’s climate is complex and responds to multiple forcings, including carbon dioxide and solar insolation,” Marcott said. “Both changed very slowly over the past 11,000 years. But in the last 100 years, the increase in carbon dioxide through increased emissions from human activities has been significant.
“It’s the only variable that can best explain the rapid increase in global temperatures.”
Results of the study, by researchers at Oregon State University (OSU) and Harvard University, are published this week in a paper in the journal Science.